P1121 nightmare

Hello everyone, I’m looking for advice regarding an ongoing issue. I performed a coolant flush on my vehicle, which was overdue, and at 250k miles I also replaced the coolant control valve as preventative maintenance using a new Toyota OEM part. I later realized how difficult bleeding the cooling system on these cars can be.

After replacing the valve, I bled the system for over 45 minutes following standard procedures and cycling the CHS pump multiple times. The car drove normally for about two weeks with no check engine light, but then the CEL began coming on intermittently. Suspecting trapped air, I maintained proper coolant levels and attempted several additional bleeding cycles. Eventually, cabin heat was lost and the CEL became constant.

I then replaced the control valve again and performed a very thorough bleed for over two hours, cycling the CHS pump repeatedly until no air appeared to remain. Despite this, after several days of driving, the check engine light returned with code P1121, still intermittent. Further bleeding attempts have not produced any air bubbles.

This has been a very frustrating experience, and I’m unsure how to resolve it permanently. If anyone has encountered a similar issue or has insight, I’d greatly appreciate your advice.